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B2-GRQ-OpGuide-v9-031020.pmd
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9.5. USING THE COMPRESSOR/LIMITER
The dynamic range (how loud we can hear to how quiet a sound we can detect) of the human ear is far
greater than the capacity of sound systems to reproduce. Although some of this equipment limitation
is at the upper extreme of the dynamic range (where too loud a signal will produce distortion), much of
the restriction occurs at the low level end, where the signal disappears below the “noise floor” of the
circuitry.
A compressor (or in its most powerful form, a limiter) is the most widely used tool for controlling dynamic
range. In the simplest terms, a compressor is designed to restrict the dynamic range of an audio
program; i.e., to make quiet signals louder, and loud signals quieter. A compressor becomes a limiter
when the compression ratio (the ratio of the input gain change to the output gain change) is so high that
the output level won’t rise above a “brick wall” ceiling regardless of how loud the input gets.
A compressor acts like an “automatic mix engineer” with a hand on the fader, and an inhumanly fast
reaction time. When the input level increases, the engineer drops the fader; when the level decreases,
the fader is raised. If the amount of fader compensation equals the variation in signal level, the output
level of the audio program will sound consistent.
9.5.1. Practical Benefits & Potential Problems of Compression
Speaker protection
A compressor will control sudden level peaks and prevent your speakers from
damage.
Perceived increase in loudness
Because peak levels are kept from rising as high as uncompressed
signals, you gain headroom for your audio program and can raise its overall
average gain. Compression is often added to the entire audio mix, both in live
sound and recording, to increase its perceived loudness.
Mix consistency
For expressive instruments or vocals, which may have a large dynamic range,
compression can help maintain consistent mix levels. So a vocal that varies
from a whisper to a scream will not disappear or stand out in the mix, relative
to other less dynamic instruments.
Like any signal processing, compression can be misused, and cause undesirable problems in the audio
signal. Some of these problems include:
Noise
If the threshold for compression is set too low, and the output gain is raised
substantially to make up for the gain loss of compression, the resulting output
signal can be noisy. This is because the input signal must be raised significantly
to produce the same output level, and the noise floor of your equipment will be
amplified unnecessarily. This problem will be exaggerated if the input signal
level to the compressor is very low (which will already degrade the signal-to-
noise ratio).
Breathing
In situations where the compression ratio is high, the threshold is low, and the
release time of the compressor is short, the noise floor will modulate up and
down as the audio signal stops and starts.
Pumping
When the compressor release time is set too long, low level signals that follow
a peak closely will disappear or fade back up as the compression releases.
Experiment with the release time to obtain the ideal setting; this will be a function
of the program content.
Over-compression
Applying too much compression to a mix can sometimes result in such evened-
out dynamics that the “life” of the music has been removed or curtailed.
Dynamic variation in music is a major component of its excitement and interest;
don’t remove them, just control them. This may be particularly true for
percussive sounds such as drums. While compressing drums can “fatten” up
the sound, setting the attack time to be longer may allow the initial strike of the
drum to have more impact.
